The invention is directed to a method for evaluating scattering signals measured with a scattered light system of a fire detector which may include a microprocessor. The scattering signals are measured at two scattering angles to determine an alarm value which is compared with an alarm threshold. The invention is also directed to a fire detector for carrying out the method.
Scattered light fire detectors typically operate with infrared light emitted by a transmitter diode at a wavelength between 800 nm and 1 .mu.m. The fire produces an aerosol which enters a measurement volume of the fire detector. The light scattered by the aerosol is measured at a backscatter angle, i.e., at an angle between 0.degree. and 90.degree., and/or a forward scattering angle, i.e., a scattering angle between 90.degree. and 180.degree.. These angles are in relation to an axis connecting the transmitter with the receiver.
The measurement of light aerosols at a forward scattering angle produces a relatively large measurement signal. Conversely, the measurement of dark aerosols at a forward scattering region produces a measurement signal which is smaller by approximately a factor 10. The magnitude of the measurement signals increases with increasing forward scattering angle. The signal produced in the backscattering regime is independent of the type of smoke and smaller than in the forward scattering regime. The difference between the scattering signals of light and dark aerosols in the backscattering regime is noticeably smaller than in the forward scattering regime.
Conventional scattered light fire detectors operating in the forward scattering regime recognize different types of dark smoke less reliably than different types of light smoke. Accordingly, the sensitivity of the fire detectors has to be adjusted to the dark smoke to safely trigger an alarm. Such a sensitivity setting, however, tends to cause a high incidence of false alarms, since the detector is too sensitive to the light smoke. In particular, a false alarm can be triggered by water vapor, cigarette smoke, vapors or fumes produced by hot grease. Conventional scattered light fire detectors are therefore not suitable for use, for example, in large kitchens or in saw mills, since the intensive vapors and dust produced in these places can be easily mistaken for light smoke.
Fire detectors operating in the backscattering regime, however, are adversely affected by particles and dust or by salt crystals which can enter the measurement volume of the fire detector and produce a significant backscatter signal, thereby producing a significant risk of false alarms.
German Pat. No. DE 42 31 088 A1 discloses a method wherein scattering signals of an aerosol which may be present in the measurement volume of a scattered light fire detector, are measured under at least two scattering angles and compared with reference data for various types of smoke which are stored in a memory. The method determines the type of smoke present in the measurement volume and sets an alarm value depending on the type of smoke. However, this method is suitable mainly for analyzing known types of smoke using the reference data stored in the memory, and may produce erroneous results for the more frequently occurring mixed fires, since such mixed fires cannot be adequately classified.